Analogue TV planning within the UK and Ireland had been broadly based on 10 UHF channel groups, each with 4 channels. There are about 48 frequency channels available in the UHF band (channels 21-68, but excluding 38 received by Radio Astronomy at Jodrell Bank). Furthermore channel 36 was used for airport radar in the UK.
DTT planning for IRL cannot be done in isolation, it must also take into account NI, England, Scotland and Wales. During planning for DTT it became apparent that a particularly difficult area would be across the Irish sea between Wales and Ireland.
DTT planning had to fit around the analogue use. Whilst ever attempt was made to retain the same receive aerial group, this was not always possible.
From the very early stages of planning for DTT in the late 1990’s it became clear that the best solution would be if Mt. Leinster and Preseli shared some channels. That enabled the best fit internally within Ireland, even though it posed challenges for Co. Wexford. The lack of service within the Irish Sea would not be relevant, and the reception of On Digital subscription services within Wexford was also not considered relevant. With a properly aligned aerial, reception of DTT from Mt. Leinster was considered possible even if frequencies were shared with the commercial multiplexes from Preseli.
An example of how engineering techniques have been used to allow very tight channel re-use can be seen where channel 53 is used for DTT at Clermont Carn near Dundalk and is also used for an analogue relay in Leitrim Co Down and at Stranraer, Scotland. During the period prior to 2013 it will not be possible to easiily receive the DTT signal from Clermont Carn within Northern Ireland.
It is intended that after the analogue services have been switched off, the DTT multiplexes be transferred to the frequencies used by the analogue services, as in many cases these will have power restrictions that are less of a contraint than the restrictions which pertain to the frequencies initally used for the DTT multiplex launched in 2010. This will be occuring at Mount Leinster (multiplexes on chns 39 and 45 move to 26 and 23), Holywell Hill (multiplex on chn 25 moves to 33) and Clermont Carn (multiplexes on chns 53 and 57 move to 52 and 56) allowing a much greater signal into Northern Ireland. These frequency channels used by Annalogue TV were included in the GE06 plan for use by DTT, so no further frequency co-ordination was required. It is merely a matter of waiting until Analogue TV is Switched Off before these channels become available.
Whilst planning had been undertaken within the context of the 6 DTT multiplexes operated in the UK (meaning a total of 10 frequencies used from many sites by the UK for analogue and digital TV services), the ITU RRC06 process allowed for equitable access to spectrum within the UHF band, i.e. an equal number of DTT multiplexes on either side of a border. The UK and IRL agreed that 8 multiplexes each be planned, which means that frequencies would be reused more often than in the 4 channel per country analogue era. A further ITU restriction was that sites with less than 250 W ERP could not be entered into the RRC06 planning process. This was in order not to make the process too complex and thus only 43 Irish sites (some new) were included. The GE06 outcome of the RRC06 was a reasonable match of DTT channels to existing analogue channels.
Since the GE06, the digital dividend has been mandated by the EC and as a consequence the channels 61 - 68 are not available for DTT from 2013. This has required a replanning of the GE06 outcome. The UK intend to operate at least 6 DTT multiplexes, Irish legislation allows for 6 multiplexes (two PSB and four assigned by the BAI). Therefore DTT services will be less likely to align with the analogue TV planning groups. As 2020 approaches there will be pressure for a second digital dividend before 2025. It may even be that the TV services will loose spectrum above 694MHz by 2020. This would mean that channels 49-60 would cease to be available for DTT. Associated with that will be even tighter reuse of frequency channels. In these circumstances reception of TV services outside their core service area cannot be guaranteed.
It is also likely that Cognitive Radio Systems will have access to the unused "white space" frequencies in an area.